Slughorn’s office

"You'd do better ... to go straight to Slughorn's office and try and get that memory from him."
--Hermione Granger to Harry Potter (HBP21)

Horace Slughorn takes up the Potions job in Harry’s sixth year. His office is located below the hospital wing (HBP18). It is described as being a very nice office, much larger than the usual teacher’s study (HBP15).

Knowing that Slughorn's office is below the Hospital Wing isn't necessarily all that helpful since the Hospital Wing is on the first floor in CS13, but on the second or third floor in OP23.